1. Field of the Invention
The present invention relates to a circuit and method for transmitting page data in a paging system and, more specifically, to a circuit and method for delaying a reference clock signal from a receiver of the Global Positioning System in order to provide the simultaneous transmission of page data in a paging system.
2. Description of the Related Art
A conventional paging system offers various wireless call services to each pager in the system. The paging system includes a plurality of base sites which offer the call service to a cell (i.e., an assigned area). Each base site receives call data from a paging terminal and sends an on-air call signal to the pager in the applicable area. Each base site should simultaneously send paging data so as to prevent a pager terminal (located in an overlapping cell area) from receiving identical signals from many base sites and to reduce the error rate of the received data.
Historically, various methods for the simultaneous transmission of page data were employed. Presently, a popular method for providing the simultaneous transmission of page data utilizes a Global Positioning System ("GPS") receiver to synchronize the transmission of page data at an absolute time. The GPS is a satellite-based radio system which utilizes various satellites orbiting around the earth for determining or guiding the location of mobile bodies such as aircraft and ships. The GPS receiver method involves transmitting multiple full waves to a satellite and measuring the phase difference between the transmitted wave and a reflected wave from the satellite. The conventional method for page data transmission using the GPS receiver involves sending synchronized page data based on absolute time information from the high-speed paging system with high-speed paging protocol.
Generally, the paging system can be divided into a paging terminal which generates page data from information received from the subscriber and a base site which receives the page data from the paging terminal and transmits an on-air frequency to the applicable pager. The base site portion of the paging system can be further divided into (1) a transmitter control portion for combining the page data (received from the paging terminal) with the appropriate protocol, synchronizing the received page data in accordance with the absolute time information (i.e., reference clock signal) from the GPS receiver, and sending the received page data to a (2) transmitter portion, wherein the received page data is modulated to an on-air frequency and then transmitted to the applicable pager.
Since the method of synchronizing the transmission of page data is basic to a paging system using a high-speed paging protocol, the transmitter controller associated with each base site must determine, beforehand, the exact time at which the page data must be sent. This determination is made based on the received page data and frame information from the paging terminal. Then, at the determined time, the transmitter controller sends the page data to the transmitter, wherein the page data is modulated to on-air frequency and then transmitted. As demonstrated above, the transmitter controller is responsible for managing all the time information and for synchronizing and transmitting the page data to the transmitter.
There are several problems associated with the actual implementation of the conventional method for simultaneously transmitting page data as described above. First, since the configuration of the simultaneous transmission of page data is based on the setup of the paging terminal, only the transmitter controller considers the timing of the page data transmission (i.e, the transmitter cannot consider these aspects). In other words, even though the transmitter controller synchronizes the transmission of the page data based on the absolute time information, the transmitter can only frequency modulate and transmit the page data received from the transmitter controller. In addition, even if the transmitter controller sends the page data to the transmitter at the correct time (i.e., the determined time discussed above), there is no way to compensate for the time delay that may occur between the time the transmitter receives and modulates the page data and the time that such data is transmitted.
Referring to prior art FIG. 1, a timing diagram is shown for the transmission of page data from the transmitter controller and the transmission of the on-air page data from the transmitter in accordance with the reference clock signal, a 1PPS (Pulse Per Second) from the GPS. Specifically, FIG. 1(a) illustrates the 1PPS signal from the GPS. FIG. 1(b) illustrates a timing diagram for the transmission of page data from the transmitter controller to the transmitter, which is synchronized to the absolute time of n seconds. FIG. 1(c) illustrates a page data transmitting clock signal which is the reference that reads in the page data from the transmitter. FIG. 1(d) denotes the timing diagram for transmission of the received page data (which is modulated to on-air frequency) from the transmitter.
In FIG. 1(b), the transmitter controller is synchronized to the absolute time of n seconds whereby the page data, as well as the transmission clock signal, is sent to the transmitter. The transmitter receives the transmitted clock signal and reads in the page data in response to the rising edge of such clock signal. The transmitter then modulates the page data to an on-air frequency and transmits it.
As shown in FIG. 1, there is a time delay associated with the reading in of page data by the transmitter, which is called the Data Fetch Delay ("DFD"), as well as a time delay associated with the process of modulating the page data (which is read in) to an on-air frequency, which is called the Frequency Modulation Delay ("FMD"). Assuming that the sum of these two delay times is the total delay time (X) of the transmitter, it is clear from FIG. 1 that the actual time of transmission of the page data from the transmitter cannot be synchronized to the absolute time n. Specifically, the modulated page data is transmitted with the time delay X (i.e., the transmitter delay time) as compared to the absolute time n.
There are various differences in the delay times among the current commercially available transmitters. Even transmitters that are assembled by the same company have slight errors between them which causes the delay times for those transmitters to vary. As a result, during actual application, even when the paging service is offered by matching various transmitters, there are differences in the time of transmission of the final on-air transmission by each transmitter in the system, thereby preventing the simultaneous transmission of page data. Consequently, in actual application, when the page data is received, the error rate is increased due to the differences in transmission times from the transmitters having different delay times.